fix(chat_templates): bind loop_messages when default_system_message is None

tests/python/test_construct_chat_template_validation.py

@@ -104,3 +104,64 @@ def test_chat_template_does_not_leak_sentinel_when_section_starts_with_it(chat_t
     )
     assert "{INPUT}" not in jinja_template
     assert "{OUTPUT}" not in jinja_template
+
+
+_SYSTEM_CHAT_TEMPLATE = (
+    "{SYSTEM}\n"
+    "### User: {INPUT}\n### Assistant: {OUTPUT}</s>"
+    "### User: {INPUT}\n### Assistant: {OUTPUT}</s>"
+)
+
+
+def _render(jinja_template, messages):
+    from jinja2.sandbox import ImmutableSandboxedEnvironment
+
+    env = ImmutableSandboxedEnvironment()
+    env.globals["raise_exception"] = lambda message: (_ for _ in ()).throw(RuntimeError(message))
+    return env.from_string(jinja_template).render(
+        messages = messages,
+        bos_token = "<s>",
+        eos_token = "</s>",
+        add_generation_prompt = False,
+    )
+
+
+@pytest.mark.parametrize("default_system_message", [None, "You are helpful."])
+def test_system_message_is_consumed_by_the_system_part(default_system_message):
+    """A caller-supplied system message must be rendered by the system part and
+    skipped by the message loop, whatever `default_system_message` is.
+
+    With `default_system_message = None` the generated template used to bind
+    `loop_messages` only inside the `{% if %}` arm. The `Fix missing
+    loop_messages` step then saw no unconditional binding, rewrote the loop back
+    to `messages`, and the system message reached the loop and tripped
+    `raise_exception`.
+    """
+    _, jinja_template, _, _ = construct_chat_template(
+        tokenizer = _SuccessFakeTokenizer(),
+        chat_template = _SYSTEM_CHAT_TEMPLATE,
+        default_system_message = default_system_message,
+        extra_eos_tokens = ["</s>"],
+    )
+    rendered = _render(
+        jinja_template,
+        [
+            {"role": "system", "content": "Be terse."},
+            {"role": "user", "content": "Hi"},
+        ],
+    )
+    assert "Be terse." in rendered
+    assert rendered.count("Hi") == 1
+
+
+def test_absent_system_message_still_renders_without_default():
+    """`default_system_message = None` with no system message in the input must
+    keep working -- the `{% else %}` arm has to bind `loop_messages = messages`."""
+    _, jinja_template, _, _ = construct_chat_template(
+        tokenizer = _SuccessFakeTokenizer(),
+        chat_template = _SYSTEM_CHAT_TEMPLATE,
+        default_system_message = None,
+        extra_eos_tokens = ["</s>"],
+    )
+    rendered = _render(jinja_template, [{"role": "user", "content": "Hi"}])
+    assert "Hi" in rendered

unsloth/chat_templates.py

@@ -2647,7 +2647,9 @@ def process(part, which, content = "message['content']"):
                 "{% set loop_messages = messages %}"\
             "{% endif %}"
         else:
-            partial_system += "{% endif %}"
+            partial_system += "{% else %}"\
+                "{% set loop_messages = messages %}"\
+            "{% endif %}"
 
         jinja_template = partial_system + jinja_template